Some frames of motorcycles include a head pipe that supports a steering shaft with a steering handle attached to an upper portion thereof, a main frame extending rearward from the head pipe, a down tube extending downward from the head pipe, a lower frame extending substantially rearward from a lower end portion of the down tube, and a pivot frame (also referred to as a swing arm bracket) extending downward from a rear end portion of the main frame, by which a rear end portion of the lower frame is coupled to the rear end portion of the main frame. Such a construction is disclosed in Japanese Laid-Open Patent Application Publication Nos. 2004-331001 and 2004-330999.
Various components are incorporated into the frame including the main frame, the pivot frame, the down tube, and the lower frame. One exemplary component is an engine mounted in the frame by a plurality of brackets. In the case of a water-cooled engine, a radiator is disposed in front of the down tube and is mounted on the down tube by brackets. Especially in motocross motorcycles, an engine guard is disposed to cover an engine from below and is mounted on the lower frame by brackets. In order to enable the main frame and the down tube to be firmly coupled to the head pipe, the main frame and the down tube are sometimes coupled to each other by reinforcing gussets.
Typically, the brackets and the gussets are manufactured separately and welded to the frame. In this case, it is necessary to weld the brackets and other components to the frame so that the components, for example, the engine, the radiator and the gussets, are incorporated into the frame with positioning accuracy. The process for welding a number of components is complicated, making it difficult to improve productivity.
Other frames of motorcycles include right and left main frames extending rearward from a head pipe, right and left pivot frames extending downward from rear end portions of the main frames, and a cross member coupling the right and left pivot frames to each other. A swing arm for rotatably supporting a rear wheel at a rear end thereof is vertically pivotally mounted at a front end portion thereof to one of the right and left pivot frames, and a suspension unit such as a shock absorber is disposed between the swing arm and the cross member and is mounted to the cross member by brackets mounted on the cross member.
In a frame thus constructed, a large load is applied to the main frames, the pivot frames, and the cross member. Since the engine, which has a large weight, is supported by the main frames and the pivot frames via the brackets, the load of the engine is applied to the respective frames. During travel of the motorcycle, a rider and the motorcycle are supported by a front suspension unit mounted to a front fork extending downward from the steering shaft and a rear suspension unit mounted between the swing arm and the cross member, a large load is applied to the cross member on which the rear suspension unit is mounted. Therefore, it is desirable to firmly couple the main frames, the pivot frames, and the cross member to each other.
Exemplary brackets by which the engine is coupled to the frames are disclosed in Japanese Published Examined Application No. Hei. 7-71953. The brackets are constructed of flat plates.
Since the engine has a large weight as described above, the brackets by which the engine is coupled to the frame are required to have strength. In order to allow the rider to easily manipulate a vehicle body during cornering, the brackets are required to have flexibility and be lightweight. Conventional brackets by which the engine is coupled to the frame are constructed of plates with a predetermined uniform thickness to have desired strength. Since the entirety of the brackets is of a predetermined uniform thickness, weight increases and suitable flexibility is difficult to obtain. If the thickness of the brackets is adjusted to provide suitable flexibility, then desired strength may be difficult to obtain.